When preparing a water-based ink for a writing instrument, particularly when preparing it in a low viscosity area (50 mPa·s or less: measured (25° C.) by means of an ELD type viscometer manufactured by Tokimec Inc.), the kind and the addition amount of materials having a large penetrating speed into paper such as surfactants having a low molecular weight of 3000 or less have so far been restricted because a non-feathering property brought about by a viscosity effect can not be expected.
However, in the case of a ballpoint pen in which metal lubricity between a ball and a tip holder is required, a surfactant has to be indispensably added. However, addition of a surfactant in an amount enough to satisfy the metal lubricity results in deteriorating the non-feathering property.
Also, in the case of a marking pen having a mechanism requiring no metal lubricity, there is the problem that the drawn lines are blurred by an effect brought about by an emulsifier adsorbed to a colored emulsion which is used as a colorant.
Further, it is possible to improve a non-feathering property by turning an ink viscosity into a non-Newtonian viscosity. Particularly in the case of a pigment ink, depending on the kind of a non-Newtonian viscosity-providing agent, the drawn lines have a good quality, but color separation is caused by settling of the pigment with the passage of time at a high temperature, or while settling of the pigment with the passage of time at a high temperature is prevented, blobbing and splitting in the drawn lines are brought about so that the drawn lines are reduced in a quality, and the writing feeling is not improved even by adding a low molecular surfactant. Thus, there exists the problem that all qualities can not be satisfied at the same time.
On the other hand, a water-based ink for a ballpoint pen comprising at least a colorant, water and polyoxyethylene styrenated phenyl ether is disclosed in Japanese Patent Application Laid-Open No. 195363/1998 as a water-based ink which inhibits blobbing and which prevents a ball holder to the utmost from being abraded by rotation of the ball to thereby secure a sufficient discharge amount of the ink and make a lot of writing possible.
However, polyoxyethylene styrenated phenyl ether added to the water-based ink for a ballpoint pen described in the patent gazette described above is used primarily as a lubricant in order to prevent the ball holder to the utmost from being abraded by rotation of the ball in the ballpoint pen tip, and it is not added for improving the non-feathering property, the writing feeling and the physical property stability of the ink.
In the present invention, a nonionic high molecular surfactant having an alkylene oxide chain as a hydrophilic group and having an average molecular weight of 3000 to 200,000 is used as described later, and it is apparently different in action and physical properties from the compound described above.